Re: Discriminants of tagged types

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Maciej Sobczak <see.my.homepage@gmail.com> writes:

> In the mean time I have solved the problem by realizing that
> "taggedness" need not be public - in my case the type was tagged,
> because it was controlled (hate to repeat it, but the interaction
> between these two unrelated language features is really annoying).
> I don't have to expose the controlled nature of the type in the public
> view and moved the "tagged" keyword to private part.
> Interestingly, this allowed me to use default values for discriminants
> *and* have the controlled type.

That doesn't sound right.  What does your example look like,
and what compiler compiled it without error?

The latest GNAT says:

     1. with Ada.Finalization; use Ada.Finalization;
     2. package Eg is
     3.    type T (X: Boolean := False) is private;
     4. private
     5.    type T (X: Boolean := False) is new Controlled with null
     record;
                                 |
        >>> discriminants of tagged type cannot have defaults

     6. end Eg;

 6 lines: 1 error

> (and this also shows that your explanation for why it cannot work is
> shaky ;-) )

Well, it would if it were true.  ;-)

> (except that now the "distinguished receiver" notation is gone...)

I'm not a big fan of that notation.  An awful lot of compiler
work for a little bit of syntactic sugar (or maybe syntactic
salt).

- Bob